Evolution__3rd_Edition

90 PART 1 / Introduction
Summary
1 Organisms produce many more offspring than can
survive, which results in a “struggle for existence,” or
competition to survive.
2 Natural selection will operate among any entities
that reproduce, show inheritance of their characteristics
from one generation to the next, and vary in
“fitness” (i.e., the relative number of offspring they
produce) according to the characteristic they possess.
3 The increase in the frequency of drug-resistant,
relative to drug-susceptible, HIV illustrates how
natural selection causes both evolutionary change and
the evolution of adaptation.
4 Selection may be directional, stabilizing, or
disruptive.
5 The members of natural populations vary with
respect to characteristics at all levels. They differ in
their morphology, their microscopic structure, their
Further reading
chromosomes, the amino acid sequences of their
proteins, and in their DNA sequences.
6 The members of natural populations vary in their
reproductive success: some individuals leave no offspring,
others leave many more than average.
7 In Darwin’s theory, the direction of evolution,
particularly of adaptive evolution, is uncoupled from
the direction of variation. The new variation that is
created by recombination and mutation is accidental,
and adaptively random in direction.
8 Two reasons suggest that neither recombination
nor mutation can alone change a population in the
direction of improved adaptation: there is no evidence
that mutations occur particularly in the direction of
novel adaptive requirements, and it is theoretically
difficult to see how any genetic mechanism could have
the foresight to direct mutations in this way.
An ecology text, such as Ricklefs & Miller (2000), will introduce life tables. For the theory
of natural selection, see Darwin’s original account (1859, chapters 3 and 4), Endler
(1986), and Bell (1997a, 1997b). Law (1991) describes the selective effects of fishing.
Travis (1989) reviews stabilizing selection. Ulizzi et al. (1998) update the human birthweight
story. Greene et al. (2000) describe another possible example of disruptive selection.
Chapter 3 in this text gave references for HIV.
Genetic variation is described in all the larger population genetics texts, such as Hartl
(2000), Hartl & Clark (1997), and Hedrick (2000). White (1973) and Dobzhansky
(1970) describe chromosomal variation. Variation in proteins and DNA will be discussed
further in Chapter 7, which gives references. The authors in Clutton-Brock
(1988) discuss natural variation in reproductive sucess.
I have concentrated on the theoretical argument against directed mutation, but
experiments have also been done. The classic one was by Luria & Delbruck (1943). It
was challenged by Cairns et al. (1988) but modern interpretations of results such as
Cairns et al. rule out directed mutation: see Andersson et al. (1998) and Foster (2000).
Two other themes are the evolution of mutation rates (see Sniegowski et al. 2000), and
the possibility that the high mutation rates of HIV could be used against them by triggering
a mutational meltdown. The underlying theory is discussed in Chapter 12 later
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